The present invention is a marine vessel for transporting mobile offshore drilling units (MODU's) from one drill site in the water to another. There are two basic types of MODUs used to drill most offshore wells: (1) bottom supported units including submersibles and jack-ups; and (2) floating units including inland barge rigs, drill ships, ship-shaped barges and semi-submersibles.
When a bottom supported MODU is on site and drilling a well, a part of its structure is in contact with the sea floor. The remainder of the rig is supported above the water. A jack-up, or self-elevating, rig is a popular design of the MODUs. Jack-ups are popular because they can drill in relatively deep water (generally up to 350 feet), provide a very stable drilling platform since part of their structure is in firm contact with the sea floor and are movable from one location to another.
Generally, two basic types of jack-ups are available. The first, a mat supported jack-up, has steel legs which are permanently attached to the "mat" at their distal ends. The mat comprises a steel frame having a plurality of ballast compartments that may be used to make the mat buoyant, neutrally buoyant, or negatively buoyant. When a mat supported jack-up is conducting drilling operations, the mat is negatively buoyant and is positioned in contact with the sea floor. In general, mat supported jack-ups are used where the sea bottom is soft and muddy. The mat distributes the weight of the rig evenly over the bottom and tends to keep it from sinking too far into a soft bottom. The mat functions similar to a stepping stone in a muddy patch of ground.
The second type of jack-up rig is the independent leg jack-up which typically has large open truss legs. The truss legs are terminated at the distal end with spud cans. Spud cans are steel cylinders with or without pointed ends. A spud can functions similar to a foundation pile that is jammed into the ground. If the sea floor is not too soft and muddy, the spud can penetrates the sea floor a short distance and supports the rig both vertically and laterally. This is referred to in the trade as "pinning." Then the rig is sufficiently loaded with ballast water to ensure the spud cans will support all anticipated work-related and environmental-related loads. In both mat supported and independent leg type jack-up rigs, the legs of the jack-up pass through openings in the barge hull (commonly referred to in the industry as a "platform") on which the MODU floats. The deck of the platform serves to support the drilling derrick, other drilling equipment, quarters, heliport and other machinery. In the trade the barge hull is frequently referred to as the "platform."
After the jack-up rig is transported to a pre-selected drilling site and is floating on the platform, the legs are jacked down in contact with the sea floor and the platform is jacked up on the legs until the platform is clear of the water's surface and well above the high waves. Large motors and a gripping system are used to raise or lower the legs as they pass through the platform. In some cases, each leg has a rack and pinion gear associated therewith. The pinion gears are powered by electric or hydraulic motors and the rack is attached to the leg.
When a jack-up is finished drilling and it is to move from one location to another, the platform is jacked down to the water line where the platform is capable of floating. The legs are then jacked upward and free from the ocean floor, wherein the jack-up rig floats on the platform.
The present invention is primarily applicable to independent leg jack-ups; therefore, subsequent discussions will concentrate on independent leg jack-ups as opposed to mat supported jack-ups. When the legs of an independent jack-up are fully elevated, the platform draws approximately 15 feet of water. If the jack-up is towed to a new location, it is capable of a maximum towing speed of approximately 5 knots. However, in rough seas the legs must be lowered to increase the stability of the jack-up. The increased drag of the legs combined with strong winds and/or currents may reduce the progress to towing speeds of less than 1 knot. Obviously, if a rig is to be moved a long distance such speeds are economically unacceptable.
In the prior art, dry lift carriers have been successfully used to move jack-ups at economical speeds. The prior art dry lift carriers are typically cargo or oil tanker ships which have had a portion of the upper decks and hull removed and replaced with a flat deck for receiving the jack-up rig. Stability columns are added on the fore and aft ends of the carrier. The dry lift carrier vessel is so named because the jack-up rig goes from a floating "wet mode" to a "dry mode" when loaded on the deck of the carrier.
Loading an independent leg jack-up on a dry lift carrier is accomplished in the following manner. The legs of the jack-up are fully elevated and the rig floats on the barge hull. The carrier is partially submerged by selective flooding of compartments in the hull and the carrier is floated into position under the platform of the jack-up. The carrier is de-ballasted by evacuating the flooded compartments and as the carrier becomes more buoyant and rises it takes on the weight of the jack-up rig on the carrier deck and the jack-up is raised from a wet mode to a "dry" mode in a manner similar to a shipyard dry dock. Alternatively in shallow water environments, the carrier may be submerged and rests on the sea floor. The jack-up rig is floated into position above the carrier. The carrier is deballasted as heretofore described. When deballasted, the carrier may transport the jack-up rig at speeds of approximately 15 knots.
Presently, there are approximately 500 jack-up rigs worldwide. Currently there are only about 12 to 15 dry lift carriers suitable for transporting jack-up rigs and of these only 6 to 8 are economically viable. As a consequence, transportation fees are quite high.
In prior art dry lift carriers, the platform of the jack-up must be supported on the deck of the carrier by timber cribbing. The cribbing is necessary to distribute the load of the platform over the deck of the carrier and for preventing damage to the underside of the platform by direct contact with the deck of the carrier. Additionally, because different jack-up rigs have somewhat different platform sizes, shapes and spacing for the legs (known in the trade as "footprints") and sizes and shapes of the platform, a customized cribbing design and load calculation must be established for each jack-up rig prior to its loading onto a carrier vessel. Additionally, customized design and calculations must be performed for sea-ties which resist lateral and transverse movement of the jack-up rig on the deck of the carrier vessel. Furthermore, many jack-up rigs do not have the capability to retract the spud cans fully into the bottom of the platform and thereby form a flat bottom. As a result it may be necessary to make one or more customized adaptations (referred to in the trade as "dimples") in the deck of the carrier. In some instances, portions of the deck must actually be cut away and bulkheads and new decking installed to accommodate the spud cans on the deck of the carrier. Therefore, a need exists for a standardized dry lift carrier that provides more efficient and economical transportation of jack-up rigs of varying footprints.